The present invention relates to the field of metrology tools for measuring semiconductor wafers and, in particular, relates to a tool that combines two complementary types of measurements into a single tool to reduce ambiguities in both types of measurements.
The semiconductor industry has a continuing interest in measuring various thin film layers formed on semiconductor wafers. A number of metrology devices have been developed for making these measurements.
One example of such an apparatus is disclosed in PCT application WO/9902970, published Jan. 21, 1999. The assignee herein has commercialized the device described in that patent application under the name OPTI-PROBE 5240. This device includes a number of measurement technologies. More specifically, the device includes a beam profile ellipsometer (BPE) (see U.S. Pat. No. 5,181,080); a beam profile reflectometer (BPR) (see U.S. Pat. No. 4,999,014); relatively conventional broad band (BB) and deep ultraviolet (DUV) spectrometers; a proprietary broad band spectroscopic ellipsometer (SE) (see U.S. Pat. No. 5,877,859) and an off-axis narrow band ellipsometer (see U.S. Pat. No. 5,798,837). All of the above-recited patents and PCT applications are incorporated herein by reference.
The above described system is particularly suited for characterizing relatively transparent films, such as silicon dioxide, on semiconductors. This measurement system is somewhat less useful when analyzing opaque films such as metals.
An optical technique which is particularly suited for measuring the thickness of very thin, metal films is X-ray reflectometry. Using a probe beam generated by a source of very short wavelength radiation, thin films can be analyzed which are opaque to both visible and UV wavelengths. One example of such a system is described in U.S. Pat. No. 5,619,548, issued Apr. 8, 1997, and incorporated herein by reference.
In an X-ray reflectometer, a probe beam of X-ray radiation is directed to impinge on the sample at an angle so that it is at least partially reflected. A sample may typically consist of a substrate covered by one or more thin metal layers. At very shallow angles, below a critical angle (xcexa8c) (as measured between the surface of the sample and the incoming ray), all of the X-ray radiation will be reflected. Typical incidence angles are very shallow, near grazing incidence, because the reflectivity falls very quickly as the angle is increased above the critical angle. As the angle of incidence of the incoming beam increases, an increasing amount of radiation will be transmitted through the top metal layer and the amount of reflected light will be reduced. Some of the radiation transmitted through the metal layer(s) will reach the interface between the metal film and the substrate and be reflected from the substrate. The radiation reflected at the interfaces among the metal film layers and the substrate will interfere, giving rise to a reflectivity curve showing interference effects. By analyzing the dependence of the reflectivity on the angle of incidence, one can independently determine both the thickness and density of the thin film layers on the sample.
The added capability offered by an X-ray reflectometer has led prior researchers to attempt to combine the measurements from an X-ray reflectometer with those of other optical measurement tools. For example, samples have been analyzed using a combination of grazing X-ray reflectometry and spectroscopic ellipsometry. (See, xe2x80x9cA new versatile system for characterization of antireflective coatings using combined spectroscopic ellipsometry and grazing X-ray reflectance,xe2x80x9d Boher, SPIE, Vol. 3741, page 104, May 1999.) Other researchers have proposed combining X-ray refletdometry with infrared spectroscopy and transmission spectroscopy. In addition, researchers have also discussed the desirability of obtaining multiple separate measurements including X-ray reflectometry, variable angle of incidence reflectometry and xe2x80x9cmiragexe2x80x9d style, photo-thermal measurements to evaluate a sample. (See, xe2x80x9cOptical and X-ray characterization applied to multilayer reverse engineering,xe2x80x9d Boudet, Optical Engineering, Vol. 37 (1), page 2175, July 1998). In this paper, the authors used the photothermal method to analyze losses from absorption.
The inventors herein have recognized that there are further advantages to combining the measurements that can be obtained from X-ray reflectometry with measurements that can be obtained from a thermal and/or plasma wave analysis. A thermal and plasma wave metrology device is marketed by the assignee herein under the name of Therma-Probe. This device incorporates technology described in the following U.S. Pat. Nos. 4,634,290; 4,346,088; 4,854,710 and 5,074,669. The latter patents are incorporated herein by reference.
In the basic device described in the patents, an intensity modulated pump laser beam is focused on the sample surface for periodically exciting the sample. In the case of a metal, thermal waves are generated, while in a semiconductor, both thermal and plasma waves are generated. These waves spread out from the pump beam spot and reflect and scatter off various features and interact with various regions within the sample in a way which alters the flow of heat and/or plasma from the pump beam spot.
The presence of the thermal and plasma waves has a direct effect on the reflectivity at the surface of the sample. Features and regions below the sample surface which alter the passage of the thermal and plasma waves will therefore alter the optical reflective patterns at the surface of the sample. By monitoring the changes in magnitude and/or phase of the reflectivity of the sample at the surface, information about characteristics below the surface can be investigated.
In the basic device, a second laser is provided for generating a probe beam of radiation. This probe beam is focused colinearly with the pump beam and reflects off the sample. A photodetector is provided for monitoring the periodic changes in the magnitude and phase of the reflected probe beam. The photodetector generates an output signal which is proportional to the reflected power of the probe beam and is therefore indicative of the varying optical reflectivity of the sample surface.
The output signal from the photodetector is filtered to isolate the changes which are synchronous with the pump beam modulation frequency. In the preferred embodiment, a lock-in detector is used to monitor the magnitude and phase of the periodic reflectivity signal. This output signal is conventionally referred to as the modulated optical reflectivity (MOR) of the sample.
This system has been used successfully to measure ion implantation levels in semiconductors. Such a system can also be used to measure thermal conductivity or thermal diffusion in a thin film layer on a sample (see U.S. Pat. No. 5,074,669, incorporated by reference). Such a system can also be used to measure characteristics of thin metalized layers (see U.S. Pat. No. 5,978,074, incorporated by reference).
Other techniques besides modulated optical reflectivity detection can be used to monitor the effects of thermal and plasma wave propagation in a sample. For example, various interferometry and mirage effects techniques have been used. The broad scope of the subject invention includes these techniques as well. (See for example, U.S. Pat. No. 6,108,087).
Since an X-ray, reflectometer measurement permits determination of both thickness and density independently, it has been recognized by the inventors herein that special benefits can be obtained by analyzing a sample using a combination of X-ray reflectometry along with a thermal and plasma wave measurement technique. More specifically, if information about the thickness and/or density of the thin film can be obtained using an X-ray reflectometer measurement, the remaining variables (such as the index of refraction and thermal conductivity) can be more easily determined with a thermal wave tool since one less variable needs to be resolved through the analysis of the data.
Accordingly it is an object of this invention to provide a measurement device that includes both an X-ray reflectometer and a thermal and plasma wave measurement module for determining the characteristics of a sample. Preferably, these two measurement modules are combined into a unitary apparatus and arranged to be able to take measurements at the same location on the wafer. A processor will receive data from both modules and combine that data to resolve ambiguities about the characteristics of the sample. The processor can either be part of the unitary apparatus or separate therefrom as long as the measurement data is transferred to the processor. In either case, the apparatus is unitary so long as the X-ray reflectometer and the thermal and/or plasma wave measurement modules are combined into a single device.